1. Field of the Invention
The present invention relates generally to programmable logic controllers and, more specifically, to a method of emulating machine tool behavior for a programmable logic controller logical verification system for manufacturing a motor vehicle.
2. Description of the Related Art
It is known that programmable logic controller code is written by controls engineers after assembly tooling designs are completed and a manufacturing process has been defined. The creation of the programmable logic controller code is mostly a manual programming task with any automation of the code generation limited to “cutting and pasting” previously written blocks of code that were applied to similar manufacturing tools. Once the programmable logic controller code is written, it is used by a programmable logic controller to operate subsequent hard tools used in the manufacture of parts for motor vehicles. The programmable logic controller code is not validated (debugged) until the hard tools are built and tried. A significant portion of this tool tryout process is associated with the debugging of the programmable logic controller code at levels of detail from a tool-by-tool level, to a workcell level and finally at a tooling or manufacturing line level.
It is also known that a manufacturing line is typically made of three to twenty linked workcells. Each workcell consists of a fixture to position product (sheet metal) and associated automation (robots) that process the product (welding). The workcell typically consists of a fixture/tool surrounded by three or four robots. The product is then transferred to the next workcell in the manufacturing line for further processing, until it exits the manufacturing line.
It is further known that the workcells for a manufacturing line can be modeled before the manufacturing line is implemented. The modeling techniques, such as Robcad from Tecnomatix and Igrip from Deneb, for the manufacturing process are limited in scope to a workcell level, due to how these type of technologies represent and manipulate three dimensional data and tool motions. This scope limitation is due to the manner in which tooling geometry is defined and the manner in which tool motions are described and displayed to a user. The geometry representation is defined through the use of (NURB) type equations, which are very exact and precise, but require intensive microprocessor calculations. The tooling and robotic motions are also microprocessor intensive in that the articulations and movements are described through the use of complex kinematic equations and solvers.
Debugging a PLC control program can broadly be grouped into two sets of conditions: one is verifying the correctness of machine logic as it applies to a workcell and local devices within the workcell; and secondly, verifying the logic of integrating the series of workcells into a manufacturing line, and especially “inter-workcell” devices like robots. Today, there is a need of using three-dimensional (3D) representations of a workcell linked to a PLC to demonstrate the correctness of the PLC logic. However, because these representations typically are full kinematic structures, the ability to scale up these representations to a full manufacturing line is severely limited by its impact on computer resources. In addition, there is no way of verifying the PLC code with the motion of a CAD model.
Therefore, it is desirable to provide a method of emulating machine tool behavior as part of a programmable logic controller logical verification system. It is also desirable to provide a method of emulating machine tool behavior with less than full kinematic structures as part of a programmable logic controller logical verification system. It is further desirable to provide a method of comparing the behavior of PLC code to accepted motion of a CAD model as part of a PLC logical verification system. Therefore, there is a need in the art to provide a method that meets these desires.